1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and, more particularly to an optical scanning device that scans a scanning surface by beams from a light source, and to an image forming apparatus including the optical scanning device.
2. Description of the Related Art
Recently, high stability relative to temperature change, high definition (high image quality), and high speed as well as cost reduction are required for image forming apparatuses such as an optical printer, a digital copying machine, and an optical plotter. These image forming apparatuses generally include an optical scanning device that scans a scanning surface by beams from a light source.
For high definition and high speed of an image forming apparatus, a method of using a plurality of beams has been used. To obtain a plurality of beams, there are two methods: (1) a method of combining a plurality of end-face emitting semiconductor lasers; and (2) a method of using an end-face emitting laser array in which a plurality of light emitting units is formed on one substrate. However, in the method (1), there are problems in that because the elements are mounted in a plurality of numbers, a light source unit becomes complicated, the number of parts increases and complicated adjustment is required. In the method (2), there is a problem that there is a difference in wavelength between the light emitting units, and individual adjustment is not possible.
A light source using a vertical-cavity surface-emitting laser (VCSEL) has been recently proposed. This type of light source can easily form several tens of light emitting units two-dimensionally on one element. Further, because light emission due to stable laser oscillation is performed by a single longitudinal mode oscillation in each light emitting unit, a difference in the wavelength between respective light emitting units is quite small.
As a representative method of achieving cost reduction of the image forming apparatus, there is a method of resinifying optical elements used for the optical scanning device. For example, if various lenses are formed of a resin material, there are merits that (1) lightening is possible, (2) low-cost molding is possible, and (3) formation of a surface having a special shape (hereinafter, also “special surface” for convenience) is facilitated. Particularly, if a resin lens having the special surface is employed, optical characteristic of the lens can be improved. As one of the special surfaces, there is a diffraction plane having a shape obtained by folding a shape of a refraction plane at an appropriate pitch. The diffraction plane can add a power higher than that of the refraction plane to the lens, thereby enabling a reduction of the number of the lenses constituting an optical system of the optical scanning device.
To realize the image forming apparatus highly stable relative to a temperature change, in the optical scanning device, (A) a method of correcting degradation of the optical characteristics due to the temperature change by combining a plurality of lenses having an opposite power to each other, and (B) a method of correcting degradation of the optical characteristics due to the temperature change by using negative dispersion of the diffraction plane have been proposed (for example, see Japanese Patent Application Laid-open Nos. 2004-126192, 2003-337295, H11-223783, 2002-214556, and 2005-215188). Particularly, along with sophistication of a forming technique of the resin material, the method of using the diffraction plane is effective for realizing the optical scanning device highly stable relative to a temperature change, with a small number of parts and low cost.
However, because the diffraction plane is very sensitive to a difference in wavelength of incident beams, there is the possibility that significant performance degradation of the optical system can occur according to magnification of the optical system and a difference in wavelength of the light source. In the method (B), therefore, the shape of the diffraction plane needs to be designed so that a power change of the diffraction plane due to wavelength variation of the light source cancels the performance degradation of the optical system. In other words, the shape of the diffraction plane needs to be designed corresponding to the optical system, and therefore generality of the lens having the diffraction plane formed thereon decreases, thereby leading to cost increase.